It’s Friday night on a packed subway train, “bodies stuck together like mollusks”, one of whom is an ageing woman quietly reading her Bible. She’s “a model senior citizen, wholesome and refined and respectable”, but when the doors slide open, the man she’s standing behind collapses on to the platform. Moments later, in the last bathroom stall at the station, she wipes poison off her dagger: since it causes paralysis seconds after entering the bloodstream she needs to be careful, “especially with the recent tremors in her hands”. 

The woman is Hornclaw, a contract killer or self-titled “disease-control specialist” at the heart of Gu Byeong-mo’s third novel. Hornclaw’s work over the past 45 years has earned her both a fearsome reputation — colleagues call her “Godmother” out of respect — and a life of solitude. She almost never knows who has commissioned the executions: the agency’s “Worryfixer” deals with the motives and the money, while longstanding arrangements with a doctor and the caretaker of a local crematorium provide neat means of disposal and medical care, if required. It’s this need that brings Hornclaw into unwitting contact with a replacement physician after “a rare bout of hand-to-hand combat” when another job goes wrong. Though she’s frightened enough to threaten the new doctor with a broken medicine bottle, he’s treated her with “a friendly, kind, devoted tone” — bizarre “when his carotid artery was about to be slashed”. Unwanted feelings of tenderness towards him, long buried, begin to haunt her as much as the memory of an old mentor whose words echo throughout the narrative, providing advice and caution. All the while, a fellow 33-year-old disease-control specialist named Bullfight has good reason to want to sabotage her. The colliding of these two plot strands makes for a claustrophobic slow-burn, a game of cat-and-mouse in which the hunter becomes the hunted: Hornclaw “feels connected to the work as if by an umbilical cord. An umbilical cord that manages barely to provide just enough nutrition before suddenly wrapping itself around your neck, choking you to death.

There are similarities with Helene Tursten’s An Elderly Lady Is Up To No Good, Oyinkan Braithwaite’s My Sister, the Serial Killer or the recent success of television shows such as Killing Eve — all of which upended traditionally male-dominated assassin stories. While an intriguing concept, there is scope here for additional character development, both of Hornclaw herself and of Bullfight, her tortured adversary.

The book’s strongest comments stem from observations about illness and deceleration, while the trauma and loneliness of these assassins’ lifestyles, especially that of our elderly protagonist, is portrayed less convincingly towards the rather formulaic climax. Nonetheless Beyong-mo’s darkly comic style addresses her primary themes with a light touch: “Trying to picture someone who has been killing people for forty-five years frying chicken or dry-cleaning clothes,” Hornclaw muses, “is like trying to imagine an old wolf incubating an egg.” This is the first of Gu Byeong-mo’s novels to be translated (by Chi-Young Kim) into English, and though Hornclaw’s story unfolds in the bustling streets of a South Korean city, the book’s real concern transcends nationality. It focuses engagingly and compassionately on the invisibility of ageing citizens, “overripe fruit”, the fears associated with retirement — of “being considered worthless even though you’re still alive”. 

The Old Woman with the Knife by Gu Byeong-mo, translated by Chi-Young Kim, Canongate £14.99/Hanover Square Press £19.99, 288 pages

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